I like to record in room but want to keep ambient background noise. For example if the room is near highway, or there is wind, or some motorcycle drive by etc, I don't want the mic to suppress those noise and want to have the capability to increase the gain of the signal to hear those noise during silence.
If you are in a situation where you are on location, recording dialogue, and the ambience is leaking into the room at an unacceptable level, you need to separate the two sound sources. You will want to try and use something that is directional inside the room to maximise "signal to ambience" ratio, and then use an external microphone - or pair of microphones to actually record the outside ambience - making as sure as possible that it is well separated from the original sound source (the dialogue).
For instance, if you are recording dialogue on a beach, you will pick up a lot of sea sounds. Use a directional boom on the dialogue, but use a pair of omnidirectional mics far away from the dialogue (so the dialogue isn't audible in the ambience mics) but it is picking up the same ambience as the spill into the dialogue mic. Then during the mix, the mixer will have full control over the dialogue level and the amount of ambience spill they have to mix into the dialogue track in order for it to sound natural.